1. Field of the Invention
The present invention relates to a bump-attached wiring circuit board having bumps of a uniform height, and to a method for manufacturing this board.
2. Description of the Related Art
It is common practice to make connections with microscopic bumps (such as those with a diameter of 50 xcexcm and a height of 30 xcexcm) when connecting a wiring circuit board and electronic elements such as semiconductor elements or liquid crystal display elements, or when connecting layers in a multilayer wiring board.
FIGS. 3A to 3E illustrate a typical method for forming bumps of this size.
First, as shown in FIG. 3A, a two-layer flexible board 33 is prepared by affixing a copper foil 32 to a polyimide film 31, and the copper foil 32 is patterned by photolithography to form a wiring circuit 34 (FIG. 3B).
Next, a cover coat layer 35 is formed by a conventional method on the wiring circuit 34 (FIG. 3C). For instance, a polyamic acid layer may be formed on the wiring circuit 34 and patterned by photolithography, and a cover coat layer 35 formed by imidation. Alternatively, a resist ink may be applied by printing.
Next, bump holes 36 are formed by irradiation with laser light in the region of the polyimide film 31 corresponding to the wiring circuit 34 (FIG. 3D), and then the cover coat layer 35 is covered with a protective film (not shown) as needed, after which microscopic bumps are formed by growing metal bumps 37 over the wiring circuit 34 exposed at the bottom of the bump holes 36 (FIG. 3E).
However, when the bump holes 36 are made by irradiation with laser light, there is variance in their open surface area due to variance in the smear amount clinging to the bottom of the bump holes 36, and this results in the problem of considerable variance occurring in the height of the metal bumps 37. This makes it difficult to achieve stable bump connection. It is particularly difficult to connect semiconductor elements all at once to a wiring circuit by ultrasonic connection. A plating pretreatment is also essential in order to improve the adhesive strength between the wiring circuit 34 and the metal bumps 37 formed on it.
The present invention attempts to solve the above-mentioned problems encountered with prior art, and it is an object thereof to provide a method for manufacturing a bump-attached wiring circuit board with which stable bump connection is possible, and there is no need for bothersome operations such as plating pretreatment.
The inventors arrived at the present invention upon discovering that bumps of a uniform height can be produced, without performing bothersome operations such as plating pretreatment, by half-etching a metal foil, whose thickness is equal to the sum of the thickness of the wiring circuit layer and a thickness corresponding to the metal bump height, down to a depth corresponding to the metal bump height, and that if a metal thin film layer composed of a different metal from the metal foil is formed on the bump formation side of the metal foil, then adhesion will be improved between the bump formation side of the metal foil and the insulating layer above, the chemical resistance of the wiring circuit board will be improved, separation between the metal foil on the bump side and the insulating layer above will be prevented, and stable bump connection can be achieved.
Specifically, the present invention is a bump-attached wiring circuit board in which a cover coat layer is formed on one side of a wiring circuit, an insulating layer is formed on the other side, and bumps that are electrically connected to the wiring circuit are formed protruding from the insulating layer, wherein the wiring circuit and the bumps are integrally formed from a single metal foil, and a metal thin film layer composed of a different metal from said metal foil is provided between the insulating layer and the side of the wiring circuit on which the bumps are formed.
Here, when the insulating layer is a polyimide film produced by the imidation of a polyimide precursor layer, it is preferable if the metal thin film layer exhibits higher adhesive force with respect to the polyimide precursor layer than to the metal foil. Favorable examples of combinations of the metal foil and metal thin film here include a metal foil that is copper foil, combined with a metal thin film of nickel, zinc, tin, or a nickel-cobalt alloy. In this bump-attached wiring circuit board, it is preferable for the cover coat layer to have a connection opening for allowing access to the wiring circuit from the cover coat side.
The present invention is also a method for manufacturing a bump-attached wiring circuit board in which bumps are formed on a wiring circuit, comprising the steps of:
(a) laminating a protective film over the bump formation side of a metal foil having a thickness equal to the sum of the thickness of the wiring circuit and the height of the bumps to be formed on the wiring circuit, and forming a wiring circuit formation etching mask on the wiring circuit formation side of the metal foil;
(b) forming a wiring circuit in a predetermined thickness by half-etching the metal foil from the side with the wiring circuit formation etching mask;
(c) removing the wiring circuit formation etching mask, and then providing a cover coat layer to the wiring circuit;
(d) removing the protective film provided to the bump formation side of the metal foil, and then forming a bump formation etching mask on this bump formation side;
(e) forming bumps of a predetermined height by half-etching the metal foil from the bump formation etching mask side;
(f) removing the bump formation etching mask, and then forming a metal thin film layer composed of a different metal from that of the metal foil;
(g) forming a polyimide precursor layer over the metal thin film layer so as to bury the bumps; and
(h) etching back the polyimide precursor layer and forming an insulating layer in a predetermined thickness by imidation.
With this manufacturing method, the wiring circuit is formed before the bumps are formed.
Further, the present invention is a method for manufacturing a bump-attached wiring circuit board in which bumps are formed on a wiring circuit, comprising the steps of:
(aa) laminating a protective film over the wiring circuit formation side of a metal foil having a thickness equal to the sum of the thickness of the wiring circuit and the height of the bumps to be formed on the wiring circuit, and forming a bump formation etching mask on the bump formation side of the metal foil;
(bb) forming bumps of a predetermined height by half-etching the metal foil from the side with the bump formation etching mask;
(cc) removing the bump formation etching mask, and then forming a metal thin film layer composed of a different metal from that of the metal foil;
(dd) forming a polyimide precursor layer over the metal thin film layer so as to bury the bumps;
(ee) etching the polyimide precursor layer and forming an insulating layer in a predetermined thickness by imidation;
(ff) removing the protective film provided to the wiring circuit formation side of the metal foil, and then forming a wiring circuit formation etching mask on this wiring circuit formation side;
(gg) forming a wiring circuit in a predetermined thickness by half-etching the metal foil from the wiring circuit formation etching mask side; and
(hh) removing the wiring circuit formation etching mask, and then providing a cover coat layer to the wiring circuit.
With this manufacturing method, the bumps are formed before the wiring circuit is formed.